The present invention relates generally to modulator-demodulators provided with plural modulator-demodulator functions having different signal processing standards, and more particularly to a modulator-demodulator which is optimized for use in a facsimile.
The modulator-demodulator, i.e. "modem," is a device for transmitting digital data using analog communications links, such as telephone lines. The communication system is specified as international standards or requirements of V-series recommendations by CCITT.
Modems are classified into low speed modems in which communication is done at a low speed up to about 300 bps, and high speed modems in which data transmission is at a high speed, recommended as CCITT V.27 and V.29 etc.; they have different modulation systems. A low speed modem is used for a control procedure which should be executed with precision even with low speed, and executes communication at the rate of 300 bps (bit per second) by means of frequency shift keying (FSK). On the other hand, a high speed modem is used for transmission of image or picture signal data at a high speed, and transmits the image data at a rate of 4800/2400 bps by means of phase shift keying (PSK) or at a rate of 9600/7200 bps by means of quadrature amplitude modulation (QAM); these are respectively specified in CCITT Recommendations V.27 ter. and V.29 for a G3 facsimile machine.
The modem incorporates modem circuits with plural standards. In particular, the modem for a facsimile machine almost necessarily incorporates plural standards since the facsimile itself incorporates plural standards of G3 (one minute machine standard, transmitting a sheet of A4 size manuscript for a time averagely shortened to one minute), G2 (three minute standard, transmitting a sheet of A4 size manuscript of manuscript for about three minutes), and G1 (six minute standard, transmitting a sheet of A4 size manuscript for about six minutes).
Moreover, it is required for the facsimile to use a modem for image signal transmission and a modem for procedure signals in different standards in order to assure the transmission control procedure; this requirement includes a concurrent operation of the plural modems with different standards for obviating erroneous sequence. Therefore, if all these standards must be satisfied by the facsimile machine as well as the modem device, the structure thereof and the control therefor will be complicated.
In particularly, in order to satisfy the requirement of concurrent operation of the modems, it was necessary to provide plural interfaces between the modems and the facsimile machine, and also necessary to provide a complicated method, in a main control section, for defining the meaning of a received signal and deciding for which of the plural modems the incoming signal is defined.
There have been proposed for the conventional facsimile the following typical three methods for discriminating which of a high speed modem signal and a low speed modem the incoming signal is defined.
(1) The first method is to concurrently operate a high speed modem H-MDM 1 and a low speed modem L-MDM 2 as shown in FIG. 1 for a facsimile host controller to check data of a line signal Sig. in a facsimile body 11. More specifically, the high speed modem H-MDM 1 demodulates the line signal in accordance with requirement T4. On the other hand, the low speed modem L-MDM 2 demodulates the line signal in accordance with the requirement T30. Both signals thus demodulated are sent to a facsimile body 11.
In response to this, whether the incoming signal is defined for the high speed modem or the low speed modem is decided by an H-processing device 111, and an L-processing device 112 within a control section located in the facsimile body 11. In accordance with this decision, a switch 113 is connected with the modem side which has produced the proper demodulated data. Thus, the selected signal is sent to a data processing section 114 so as to perform a proper facsimile communication operation.
(2) The second method is to carry out the discrimination in a sequence.
When the line signal Sig. has data, the signal for one modem which can easily be found for a short time is earlier subjected to signal receipt processing; if the processing result for a predetermined time is NG, the signal receipt processing is switched to the other modem. In the example shown in FIG. 2, the high speed modem H-MDM 1 and the H-processing device 111 are first operated; if the processing result for a predetermined time is NG, the signal reception processing is changed to the low speed modem L-MDM 2 and L-processing device 112 by connecting switches 113, 115, 116 and 117 as indicated by arrows. This arrangement, in which a common input/output terminal for plural modems of the high speed modem and the low speed modem is exchanged by the switches, can be fabricated with a comparatively low cost.
(3) The third method is to use the result of detecting a training signal with a high speed modem requirement.
The method of (3) is to detect that the received signal is a high speed modem signal using a training signal of about several milli-seconds to several hundreds milli-seconds added to the head of the received high speed modem signal.
The conventional high speed modem mentioned above is liable to be influenced from noise and several kinds of waveform distortion within the telephone lines. In method (3), therefore, the modem exchange cannot be carried out properly as the case may be; even if this is successful, the signal for the modem after exchange cannot be obtained immediately.
The prior art of using the result of detecting the training signal is disclosed in ROCKWELL 1985 DATA BOOK p. 7-7 to 7-19, R96 FAX 9600 BPS FACSIMILE MODEM.
The form of realizing a modem has been miniaturized and economized, namely, in structure, from a box type in which a large number of electronic circuit boards are packaged inside a large box to a card type in which the modem is packaged on a sheet of electronic circuit board, and in function, from a type of simultaneously using plural modems to a unified type of executing plural modem functions at a common input/output terminal. Either type, however, sacrifices concurrent operations of the plural modem functions and so is uncertain in switching the modem function. The above prior art also suffer from such a disadvantage.